1. Technical Field
The present invention relates to a charged particle beam irradiation apparatus for irradiating a charged particle beam.
2. Description of the Related Art
A charged particle beam irradiation apparatus for irradiating a charged particle beam is disclosed in the related art. The charged particle beam irradiation apparatus disclosed in the related art is an apparatus for performing treatment by irradiating a tumor within the body of a patient with a charged particle beam. The charged particle beam irradiation apparatus includes a cyclotron that generates a charged particle beam by accelerating a charged particle, a scanning electromagnet that scans the charged particle beam, and a degrader that is provided on a downstream side of the scanning electromagnet in the scanning direction of the charged particle beam and adjusts the range of the charged particle beam by reducing the energy of the charged particle beam. The degrader is a plate-like member that extends with a fixed thickness in a scanning direction perpendicular to the irradiation direction of the charged particle beam.
The degrader of the charged particle beam irradiation apparatus in the related art has a rectangular cross-section in the irradiation direction of the charged particle beam (for example, refer to FIG. 5). When such a degrader is used, a charged particle beam made to go straight without being deflected by the scanning electromagnet is perpendicularly incident on the degrader. Accordingly, the passing distance of the charged particle beam within the degrader is equal to the thickness of the degrader (thickness of the degrader in the Z direction in FIG. 5). On the other hand, in the case of a charged particle beam deflected by the scanning electromagnet, the passing distance of the charged particle beam within the degrader is increased if the deflection angle is large. In this case, since the energy of the charged particle beam decreases as the passing distance of the charged particle beam within the degrader increases, the range of the charged particle beam becomes short.